Cracking research

Researchers are applying the same basic technique seismologists use to measure earthquakes for a new medical technology that promises to prevent stress fractures. The technology works by detecting the formation of tiny cracks in bones.



The crack formation generates waves similar to those created by earthquakes. Researchers at Purdue University and the University of Toledo have collaborated to create a prototype device that could be used to monitor the formation of these “microcracks” in bones that can lead to hairline stress fractures unless detected in time.



‘The goal is to create a wearable device that would alert the person when a stress fracture was imminent so that they could stop rigorous physical activity long enough for the bone to heal,’ said Ozan Akkus, an associate professor in Purdue’s Weldon School of Biomedical Engineering.



The system records “acoustic emission data,” or sound waves created by the tiny bone fissures. The same sorts of acoustic emissions are used to monitor the integrity of bridges, other structures and mechanical parts like helicopter turbine blades.



‘I asked, why not use the same approach to study stress fractures?’ Akkus said.



Such a technology could help prevent serious stress fractures in racehorses and those who perform in situations that cause undue stress to bones, such as soldiers, athletes and dancers.



Cracks form when collagen fibres in bone fail, producing sound waves that cause a rippling motion on the skin’s surface.



‘This is the same thing that happens during an earthquake, but on a microscopic scale and at a higher frequency,’ Akkus said. ‘Instead of an earthquake-size opening, these cracks are about a tenth of a millimetre wide.’



Accumulating cracks sometimes cause “spontaneous fractures” that occur without warning, afflicting the young and old alike, including athletes and elderly people suffering from osteoporosis.



A major factor in the crack formation is the dynamic process bones use to continually rebuild themselves. When bone is damaged, specialized cells bore tunnel-like holes to remove the damaged tissue and then fill in the resulting cavity with new bone.



Diagnosis



One reason it’s difficult to diagnose the hairline fractures is because they are caused by the gradual accumulation of microscopic cracks, which are not detectable with conventional imaging technologies.



The researchers are developing the monitoring technique by studying crack formation in pieces of bone from human cadavers that are placed in a machine that continually bends the bone until it cracks.



Akkus is working with researchers at the University of Toledo to develop a wearable prototype that will record crack-formation data, which could be downloaded to a portable digital assistant (PDA), for review by medical professionals. Such a device could immediately alert the person by sounding an alarm, and the data could then be scrutinised by a doctor.



Sensors made of a “piezoceramic” material generate electricity when compressed by a force, such as the vibration created by seismic waves resulting from crack formation.



‘Recently, flexible polymer-based sensors have appeared on the market, and these could be incorporated into athletic apparel, such as running shoes and exercise tights to monitor areas most susceptible to fractures,’ Akkus said. ‘Ultimately, we would like to do real-time monitoring of damage activity and learn how to distinguish between a small crack and a more structurally threatening defect.



‘There are different types of cracks that occur, and it’s important to be able to distinguish among them so that we can determine how serious the damage is.’



To distinguish the difference between the various types of cracks, researchers are integrating “pattern recognition” software and earthquake models, working with Robert Nowak, a Purdue professor of earth and atmospheric sciences.



‘One challenge will be to learn when damage is serious enough that you should stop exercising,’ Akkus said. ‘You don’t want to give a professional athlete a premature warning.’